Images to be delivered on digital broadcasts, those to be stored in media such as DVDs and hard disks, and the like are compressed by various coding systems. The object for such compressions is to decrease the burden on a transmission band, increase the transmission speed, decrease the memory size, or the like.
There are various standards for image coding system, such as MPEG2 and H.264. And there is a case where the coding system is changed for the purpose of reducing the amount of codes in a coded image that is inputted, or the like. A transcoder once decodes the inputted coded image. Then, the transcoder encodes the decoded image by a different coding system (or the same coding system) again. Thus, the transcoder controls a bit rate of an output stream.
The following Patent Document 1 discloses a technique for calculating a quantization step value of an output stream in a transcoder. In the following Patent Document 1, the quantization step value is adjusted in consideration of the degree of dispersion of pixels in a macroblock of a decoded image and a motion evaluation value.
[Patent Document 1] Japanese Patent Application Laid Open Gazette No. 2008-42426
In a case where the quantization step value of the output stream is controlled in order to keep the bit rate constant or reduce the bit rate, the variation of the quantization step may increase depending on the condition of an input stream or the calculation condition. When the variation of the quantization step becomes large, a rate control becomes unstable and this causes degradation of image quality.